code: def _downgrade_v3(op):
      """
      Downgrade assets db by adding a not null constraint on
      ``equities.first_traded``
      """
      op.create_table(
          "_new_equities",
          sa.Column(
              "sid",
              sa.Integer,
              unique=True,
              nullable=False,
              primary_key=True,
          ),
          sa.Column("symbol", sa.Text),
          sa.Column("company_symbol", sa.Text),
          sa.Column("share_class_symbol", sa.Text),
          sa.Column("fuzzy_symbol", sa.Text),
          sa.Column("asset_name", sa.Text),
          sa.Column(
              "start_date", sa.Integer, default=0, nullable=False
          ),
          sa.Column("end_date", sa.Integer, nullable=False),
          sa.Column("first_traded", sa.Integer, nullable=False),
          sa.Column("auto_close_date", sa.Integer),
          sa.Column("exchange", sa.Text),
      )
      op.execute(
          """
          insert into _new_equities
          select * from equities
          where equities.first_traded is not null
          """,
      )
      op.drop_table("equities")
      op.rename_table("_new_equities", "equities")
      # we need to make sure the indices have the proper names after the rename
      op.create_index(
          "ix_equities_company_symbol",
          "equities",
          ["company_symbol"],
      )
      op.create_index(
          "ix_equities_fuzzy_symbol",
          "equities",
          ["fuzzy_symbol"],
      )
  
file_path: catalyst/catalyst/assets/asset_db_migrations.py
imports: from functools import wraps
  from alembic.migration import MigrationContext
  from alembic.operations import Operations
  import sqlalchemy
  from toolz.curried import do, operator
  from catalyst.assets.asset_writer import write_version_info
  from catalyst.errors import AssetDBImpossibleDowngrade
  from catalyst.utils.preprocess import preprocess
  from catalyst.utils.sqlite_utils import coerce_string_to_eng
label_desc: Use image node_id 6 for calling a global function with example usage: _downgrade_v3(op) without return types